Epitaxial growth technique is conventionally used to produce a semiconductor device such as a power device (e.g., IGBT (Insulated Gate Bipolar Transistor)) requiring a relatively thick crystalline film.
In the case of vapor phase epitaxy used in an epitaxial growth technique, a substrate is placed inside a film-forming chamber maintained at an atmospheric pressure or a reduced pressure, and a reaction gas is supplied into the film-forming chamber while the substrate is heated. As a result of this process, a pyrolytic reaction or a hydrogen reduction reaction of the reaction gas occurs on the surface of the substrate so that an epitaxial film is formed on the substrate. The gas generated by the reaction, as well as the gas not used, is exhausted through the outer portion of the chamber. After the epitaxial film is formed on the substrate, the substrate is then carried out from the chamber. Another substrate is then placed into the chamber, and then an epitaxial film will be formed on that substrate.
In order to produce a thick epitaxial film in high yield, a fresh reaction gas needs to be continuously brought into contact with the surface of a uniformly heated substrate to increase a film-forming rate. Therefore, in the case of a conventional film-forming apparatus, a film is epitaxially grown on a wafer while the wafer is rotated at a high speed (see, for example, Japanese Patent Application Laid-Open No. 2009-170676).
In a conventional film-forming apparatus, a rotating unit is positioned in a film-forming chamber, and a substrate is positioned on a ring-shaped holder arranged on the top-surface of the rotating unit. A resistive heater functioning as an inner heater is positioned below the holder. This heater is a spiral-shaped structure made from carbon. Terminals and wires for supplying current, and electrodes for supporting the heater are arranged in a rotating shaft, the rotating shaft is a part of the rotating unit.
The substrate is heated via radiant heat from the inner heater. However the temperature of the outer periphery of the substrate is lower than the inner periphery, as the reaction gas rapidly flows at the outer periphery of the substrate and the radiation heat is lost as a result of cooling water in the outer wall of the film-forming chamber used to cool the apparatus. Therefore the inner heater consists of an in-heater and an out-heater, and the temperature of the out-heater is higher than the temperature of the in-heater thereby maintaining the substrate at a consistent temperature. Japanese Patent Application Laid-Open No. 2009-170676 mentions an outer heater arranged between the rotating unit and the inner wall of the film-forming chamber thereby extending the life of the out-heater.
In recent years, attention has been given to SiC (silicon carbide) as a semiconductor material to be used in high-voltage power semiconductor devices. SiC is characterized in that its energy gap is two or three times larger, and its dielectric breakdown field is about one digit larger than that of a conventional semiconductor material such as Si (silicon) or GaAs (gallium arsenide).
When mono-crystalline film made from SiC is formed on the substrate, the substrate will be heated to over 1500° C. Japanese Patent Application Laid-Open No. 2009-170676 mentions a method for heating from the bottom side of the substrate but there is a difficulty in maintaining this high temperature.
Another method for heating from top and bottom sides of the substrate is known. According to this method, the substrate is heated by a conventional inner heater, and an upper heater is arranged in the side wall of the upper part of the film-forming chamber. The inner heater is a resistive heater, and the upper heater is a high-frequency induction heater.
In the conventional method for heating the back side of the substrate, when the temperature of the substrate is over 1500° C., the temperature of the heater needs to be over 2000° C. In the resistive heater, a capacity for heat-resistance of the material used for electrically connecting the heater to the electrode is low, therefore it is difficult for the properties of the heater to be maintained at the above-mentioned high temperature. Accordingly, the upper heater is provided to heat the substrate in collaboration with the inner heater, further, the high-frequency inducted heater is used as the upper heater. Therefore, the substrate can be heated to a temperature over 1500° C.
In the high-frequency inducted heater, the heating effectiveness depends on the distance from the substrate. The temperature of the substrate is controlled using this property. Specifically, the heating temperature is controlled by adjusting the position and height of the heater coil, which is a part of the heater. However, with this method it is difficult to perform precise control of the temperature. That is, the resistive heater can only perform a minor degree of control of the temperature of the substrate, but the temperature can only be controlled to a much larger degree when the resistive heater and the high-frequency inducted heater are used.
Also, the upper heater cannot efficiently heat the substrate positioned below the upper heater as the heat dissipates into the upper part of the chamber. Moreover the construction of the upper part of the film-forming chamber, specifically a liner between the inner wall of the film-forming chamber and space in which the film-forming process performs, would crack because of the temperature difference between the inner wall and the space, by the upper heater.
The present invention has been made to address the above issues. That is, an object of the present invention is to provide a film-forming apparatus and a film-forming method that can prevent cracks in a liner and can efficiently and precisely heat a substrate, and can control the temperature of the substrate.
Other challenges and advantages of the present invention are apparent from the following description.